1. Field of the Invention
The present invention relates to a minimally-invasive system and method for monitoring analyte levels in a patient. More particularly, the present invention relates to a system and method employing a device which includes a micro probe functioning as an active electrode and a auxiliary electrode surrounding at least a portion of the active electrode, arranged to be placed against the skin of a patient to detect analyte levels in the patient with minimal pain and damage to the patient's skin.
2. Description of the Related Art
People having diabetes must monitor their blood glucose level on a regular basis to assure that their blood glucose level remains within normal limits necessary to maintain a healthy living condition. Low glucose levels, known as hypoglycemia, can cause mental confusion and, in more extreme instances, coma and ultimately death. On the other hand, high blood glucose levels, known as hyperglycemia, can cause chronic symptoms such frequent urination and thirst, and if sustained over long periods of time, can result in damage to blood vessels, eyes, kidneys and other organs of the body.
Some people having mild diabetes can regulate their blood glucose levels through diet. However, people having moderate or severe forms of diabetes must take insulin to sustain acceptable blood glucose levels.
Conventional methods of monitoring blood glucose levels directly monitor the concentration of glucose in a small sample of blood taken from the person. Accordingly, if the person wishes to test his or her blood glucose level, the person can use a small needle or lance to puncture, for example his or her fingertip and drain a droplet of blood into the sampling device. However, this invasive method is painful to the person. Moreover, precautions must be taken sterilize the area in which the puncture is made, as well as the puncturing instrument, so that a pathogen is not introduced into the person's bloodstream. These methods can also be somewhat messy and unsanitary, and somewhat time consuming.
As an alternative to the conventional invasive techniques, miniaturized glucose sensing needles have been developed over the past several years. These types of devices typically include a metal substrate with an enzyme as an active electrode and an adjacent metal substrate that serve as the return and reference electrodes. The enzyme, typically glucose oxidase, catalyzes the oxidation of glucose, and the byproducts of the reaction are measured electrochemically at the active electrode. The electrochemical measurement is affected by imposing an electrical potential between the active and the counter/reference (auxiliary) electrodes. At a particular potential, electric current begins to flow as a consequence of the chemical reaction at the electrodes. The current is related to the concentration of the electro-active species, which is in turn governed by the amount of glucose in the test medium. In the case of conventional glucose strips, the test medium is capillary blood; in the case of implantable electrodes, the medium is tissue.
These devices are typically macroscopic or, in other words, more than 200 microns in diameter and often a centimeter or more in length. Accordingly, these devices are invasive, because they can penetrate the skin up to one centimeter deep. Additionally, these devices typically employ conventional needles, wires and multi-layer plastic substrates which require complicated multi-step manufacturing processes that are both time consuming and expensive. Examples of known glucose sensing devices are described in U.S. Pat. Nos. 4,953,552, 5,680,858 and 5,820,570, and in PCT publication WO 98/46124.
Maximizing the active electrode area increases the current response of the system. Especially in the case of the implantable system, the active electrode area is small—often ten-fold smaller than strip-based electrodes. Furthermore, the active and return/reference electrodes often share the same substrate—further limiting the available active area. One advantage of the invention described herein is that the return electrode is separated from the active electrode substrate and can be positioned on the surface of the skin, at least partially surrounding the minimally-invasive working electrode. This configuration allows maximum usage of the active electrode, and it permits the use of a large external return electrode. Both aspects improve the signal and performance of the system, while maintaining the small minimally invasive, pain-free format of the design.
Accordingly, a need exists for an improved minimally invasive system for monitoring analyte levels in patients.